In the production of optical lenses, grinding and polishing operations are generally carried out utilizing tools known as "blocks" and "laps". The block is the tool in which the lens blank is secured, normally with a low melting alloy, and the lap (rectangular or round) is the tool upon which the surfacing operation is performed. In both instances, the tools must be very hard, wear resistant and durable, and they must have a high degree of dimensional stability and machineability. Thus, it is essential in producing precision lenses that the surfaces used for mounting of the tool, and for engagement of the lens blank, accurately correlate to one another, to the lens blank, and to the machine or device in which the tool is mounted. The tool must not be subject to undue damage in normal use, nor to undue wear from the abrasives with which certain of the surfacing operations are carried out, and it must be capable of withstanding secure clamping or support by other means in the mechanism involved, without damage or distortion. Normally, the blocks and laps are produced with standard curvatures, and are then subsequently machined to precise specifications; consequently, it is imperative that the tool be readily adapted to such operations. Finally, if the heat generated during the surfacing operations is not dissapated, it can ultimately cause surface warpage, and this is particularly true in the instance of plastic lenses; although flooding with an aqueous coolant effectively dissipates a large amount of the heat, still the thermal conductivity of the tools (and especially the lens block) is an important consideration.
Typically, lens finishing laps and blocks have been produced from metals; iron and aluminum are employed most prevalently, but bronze and brass are also utilized to some extent. The metal tools are, however, relatively expensive and heavy. The cost of such a tool has demanded careful handling and maintenance; moreover, relatively complex constructions, such as the utilization of hard metal inserts in a sintered metal block, have been employed to minimize costs. Weight, of course, is disadvantageous not only from the standpoint of handling and transportation, but perhaps more significantly from the standpoint of producing inordinate levels of wear in the associated equipment. For example, a typical orbital surfacing machine utilizes a so-called "wobble bearing" in the lap supporting mechanism. The weight of the lap obviously has a profound effect upon the load to which the wobble bearing is subjected, and the use of conventional metal laps often causes such wear as to require frequent replacement of the bearing.
In an effort to avoid the cost of other disadvantages of utilizing metal tooling, attempts have been made to substitute parts fabricated from synthetic resinous materials. Typical of such an attempt is the approach described in U.S. Pat. No. 2,426,215, to Hicks, wherein a thermoplastic synthetic resin molding compound is described for use in producing an abrading lap for cold working of optical materials. The resins disclosed include polystyrene, methylmethacrylate polymer, polyvinyl acetate and polyvanylidene chloride resins. As a practical matter, such laps are found to be deficient and not commercially feasible, due largely to the poor machineability that is afforded by the resins proposed; thermoplastic tools of this sort may also be unduly susceptible to the effects of thermal energy generated during the surfacing operations.
Accordingly, it is an object of the present invention to provide a novel, monolithic tool for use in surfacing of optical lenses, which is relatively inexpensive and relatively light in weight.
It is a more specific object of the invention to provide such a tool which is produced from an epoxy resin, and which is uniformly hard, durable, dimensionally stable, and machineable, and which may exhibit enhanced thermal and electrical conductivity.
It is also an object of the invention to provide a novel method for producing such a tool having uniform properties, from an epoxy casting compound, which method may be carried out quickly and conveniently, and at relatively low cost.
A more specific object of the invention is to provide such a novel method wherein silicone molds may be employed in a centrifugal casting technique, and wherein the casting compound is formulated to afford facile and efficient casting and rapid curing, to cause a minimum level of attack upon the silicone mold members, and to shrink to a minimum extent during the molding operation, and to produce a tool that accurately reproduces the dimensions and configuration of the mold cavity.